Simply put, they are both true to their nature; they don't play games and use tricks. The fisherman plays by the rules (an important concept for Hemingway) and fights a fair fight; the fish does the same. It is true to its nature and gives Santiago all he can deal with. They both fight to exhaustion, and it is never clear until the end who is going to win. Sadly, neither of them can win. Santiage "wins" the fish, but the Sharks beat both of them, devouring the fish on its way back. But this is Hemingway's point. You are never guaranteed a "win" no matter how well you play, so don't worry about it. All you can control is how you "play the game" --- how honest you are to yourself and to what you undertake.
So Santiago and the fish are both the same --- honest, true to their nature, and beaten.